This application relates to the operation of re-programmable nonvolatile memory systems such as semiconductor flash memory systems, and, more specifically, to systems and methods of configuring such systems.
Solid-state memory capable of nonvolatile storage of charge, particularly in the form of EEPROM and flash EEPROM packaged as a small form factor card, has become the storage of choice in a variety of mobile and handheld devices, notably information appliances and consumer electronics products. Conventional mass storage, based on rotating magnetic medium such as hard drives and floppy disks, is unsuitable for the mobile and handheld environment. On the other hand, flash memory, both embedded and in the form of a removable card is ideally suited in the mobile and handheld environment because of its small size, low power consumption, high speed and high reliability features.
Flash EEPROM is similar to EEPROM (electrically erasable and programmable read-only memory) in that it is a non-volatile memory that can be erased and have new data written or “programmed” into their memory cells. Both utilize a floating (unconnected) conductive gate, in a field effect transistor structure, positioned over a channel region in a semiconductor substrate, between source and drain regions. A control gate is then provided over the floating gate. The threshold voltage characteristic of the transistor is controlled by the amount of charge that is retained on the floating gate. That is, for a given level of charge on the floating gate, there is a corresponding voltage (threshold) that must be applied to the control gate before the transistor is turned “on” to permit conduction between its source and drain regions. In particular, flash memory such as Flash EEPROM allows entire blocks of memory cells to be erased at the same time.
Nonvolatile memory devices are also manufactured from memory cells with a dielectric layer for storing charge. In addition to charge storage memory, other forms of nonvolatile memory may be used in nonvolatile memory systems. For example Ferroelectric RAM (FeRAM, or FRAM) uses a ferroelectric layer to record data bits by applying an electric field that orients the atoms in a particular area with an orientation that indicates whether a “1” or a “0” is stored. Magnetoresistive RAM (MRAM) uses magnetic storage elements to store data bits. Phase-Change memory (PCME, or PRAM) such as Ovonic Unified Memory (OUM) uses phase changes in certain materials to record data bits. Various other nonvolatile memories are also in use or proposed for use in nonvolatile memory systems.
Nonvolatile memory systems, such as flash memory systems are commonly provided in the form of a memory card or flash drive that is removably connected with a variety of hosts such as a personal computer, a camera or the like, but may also be embedded within such host systems. A common memory system comprises one or more memory chips that are mounted on a printed circuit board, along with a memory controller chip, and enclosed in a housing to form a memory card or Universal Serial Bus (USB) flash drive that has a physical interface that allows it to be connected with a range of host devices.